The present invention relates to a wavelength division multiplexing optical fiber transmission line that reduces signal deterioration due to dispersion, and allows long-distance transmission to be made even at a high-speed transmission rate.
As a rule, in an optical fiber transmission system with a wavelength division multiplexing that is referred to as a WDM, excellent and yet uniform transmission quality is required for all channels in which a wavelength is divided and transmitted. On the other hand, as an optical fiber configuring a signal transmission line has wavelength dependency (dispersive slope) normally of differing in a dispersive amount responding to the wavelength, it is well-known that different transmitting characteristics are shown channel by channel to be used (for each wavelength to be used).
From such a background, so as to realize uniform transmission quality for all channels in the optical fiber transmission system with the wavelength division multiplexing, dispersive flatness of the transmission optical fiber (that dispersion is less dependent on the wavelength) is required. So as to realize this dispersive flatness, it is known that a dispersion management transmission line 19 shown in FIG. 9 (A) is effective. This dispersion management transmission line 19 has a positive dispersion fiber 21, of which a wavelength dispersion value is positive, and a negative dispersion fiber 22 for compensating for accumulated dispersion and the dispersive slope, which are accumulated in this positive dispersion fiber 21, arranged in turn. Furthermore, an optical repeater 18 is arranged in a front stage of the dispersion management transmission line 19, and an optical repeater 20 is arranged in a rear stage of the dispersion management transmission line 19.
Employment of such a configuration allows a dispersion characteristic to be flattened to some degree. In the event that the accumulated dispersion caused by the positive dispersion fiber 21 is large, however, a phenomenon occurs that a nonlinear component is generated due to an interaction between this accumulated dispersion and the negative dispersion fiber 22, thus giving rise to ultimate transmission deterioration.
This phenomenon will be specifically explained by referring to FIG. 9 (B).
In FIG. 9 (B) are illustrated as a map accumulated dispersion amounts in the longitudinal direction of a span of the optical repeater 18, the dispersion management transmission line 19, and the optical repeater 20. Furthermore, conceptual time waveforms at typical several points on the map are also illustrated.
F point shown in a graph of the accumulated dispersion of FIG. 9 (B) corresponds to an output region of the optical repeater 18 (an input region of the positive dispersion fiber 21). Also, G point corresponds to a connection region between the positive dispersion fiber 21 and the negative dispersion fiber 22. Furthermore, H point corresponds to an output region of the negative dispersion fiber 22 (input region of the optical repeater 20).
In a characteristic f at point F, output signal waveforms at times t1 and t2, which correspond to wavelength multiplexing channels (for example, two channels), are shown in a time axis t. This characteristic f assumes a normal and beautiful waveform because of a state that the signal, which was just amplified and compensated by the optical repeater 18, was input into the positive dispersion fiber 21.
On the other hand, a characteristic g at G point is in an attenuated state due to the fact that the output of the optical repeater 18 passes through the positive dispersion fiber 21, and yet is set into a state that a leading edge of the waveform was gently spread due to a component of the accumulated dispersion to be generated by passing through the positive dispersion fiber 21. It is in a state that the waveforms are in a heavily overlapped between two bits, thus causing a large nonlinear effect (self-phase modulation) to occur.
Such a signal at G point becomes a characteristic h that is in a state that compensation was made for the accumulated dispersion and the dispersive slope by passing through the next-stage negative dispersion fiber 22. Waveforms at this H point became distorted because its original waveform having the ideal state (characteristic h) was not correctly reproduced, and thus, transmission quality became deteriorated. Such deterioration of transmission quality is due to generation of a component that is beyond of the capability of the compensation in the event of targeting a large accumulated dispersion amount because the compensation is made only one time in one dispersion management transmission line 19.
For this reason, for example, a technology described below was disclosed in JP-P2001-91761A. One dispersion management transmission line has a positive dispersion fiber section, of which the wavelength dispersion is positive, and a negative dispersion fiber section, of which the wavelength dispersion is negative, arranged alternatively in plural. And not by making component compensation only one time, but making it step by step for transmission deterioration caused by an interaction between the nonlinear effect (self-phase modulation), which occurs in the optical fiber, and the wavelength dispersion, deterioration of the signal to be finally obtained is reduced.
Also, a technology described below was disclosed in JP-P1999-331074A as well. An optical fiber transmission line is split into a plurality of blocks for connection. So as to optimize dispersion compensation for each of this plurality of the blocks (to nullify a dispersive error), its blocks are caused to actively have a residual dispersion amount to a certain degree, and a block dispersion compensator is provided in a final region (region of the last block out of a plurality of the blocks) in which a receiver etc. is installed. A total residual dispersion value is nullified in the optical fiber transmission line by means of this block dispersion compensator.
However, the conventional wavelength division multiplexing optical fiber transmission line is configured so that the dispersion management transmission line to be arranged among a plurality of the optical repeaters has the positive dispersion fiber section and the negative dispersion fiber section alternatively arranged in plural. And, signal strength of the optical signal is high in an exit of the front-stage optical repeater (an input region to the fiber) because it has just been amplified, and on the other hand, the signal strength is set into a low state in an entrance of the next-stage optical repeater (an output region of the fiber) because the optical signal was subjected to passage loss of the fiber.
Also, in the event that the signal strength is high, the nonlinear effect in the fiber occurs greatly, whereby as a rule, in configuring the span by alternatively combining positive and negative dispersion, the positive dispersion fiber, which has a large core diameter and is less influenced by the nonlinear effect, is arranged in the front-half portion of the span (in the vicinity of the exit of the optical repeater in the front-stage portion). Also, the negative dispersion fiber, which has a small core diameter and is greatly influenced by the nonlinear effect, is arranged in the latter-half portion of the span (in the vicinity of the entrance of the optical repeater in the rear-stage portion).
And, so as to prevent transmission deterioration, when the span is configured with the alternate number of the fibers for dispersing positively and negatively increased, the negative dispersion fiber of which nonlinearity is large results in being arranged in the front-half portion of the span as “a negative dispersion fiber of which the nonlinearity is large”. For this reason, an influence becomes large of the nonlinearity caused by the accumulated dispersion in the entirety of the above span, and conversely, the problem occurs that waveform deterioration is incurred.
Even though such a problem does not give rise to a special issue practically in the event that a transmission rate is 10 G b/s or something like it, or not more than this, it becomes a serious problem specially in the recent years that making the transmission rate much faster is under progress, as is the case with 40 G b/s of the next generation. That is, the foregoing conventional system, in which sufficient characteristics were not attained for the dispersive flatness, the low nonlinearity, the span accumulated dispersion, etc. in the transmission system having a high-speed transmission rate, for example, such as 40 G b/s, was not enough for a long-distance transmission.
Thus, an objective of the present invention is to provide a wavelength division multiplexing optical fiber transmission line that can reduce signal deterioration caused by an interaction between the accumulated dispersion and the nonlinear effect, and allows long-distance transmission to be made even at a high-speed transmission rate.